Optimal control simulation is used to examine the control mechanisms in the photodissociation of phenol within a two-dimensional, three-electronic-state model with two conical intersections. This model has two channels for H-atom elimination, which correspond to the π 2 and 2 states of the phenoxyl radical. The optimal pulse that enhances 2 dissociation initially generates a wave packet on the S1 potential-energy surface of phenol. This wave packet is bifurcated at the S2 - S1 conical intersection into two components with opposite phases because of the geometric phase effect. The destructive interference caused by the geometric phase effect reduces the population around the S1 - S0 conical intersection, which in turn suppresses nonadiabatic transitions and thus enhances dissociation to the 2 limit. The optimal pulse that enhances S0 dissociation, on the other hand, creates a wave packet on the S2 potential-energy surface of phenol via an intensity borrowing mechanism, thus avoiding geometric phase effects at the S2 - S1 conical intersection. This wave packet hits the S1 - S0 conical intersection directly, resulting in preferred dissociation to the π 2 limit. The optimal pulse that initially prepares the wave packet on the S1 potential-energy surface (PES) has a higher carrier frequency than the pulse that prepares the wave packet on the S2 PES. This counterintuitive effect is explained by the energy-level structure and the S2 - S1 vibronic coupling mechanism.